A coordinated turn for a rotary wing aircraft, e.g., a helicopter, is defined as a banked turn where the body of the aircraft is tangential to a curvilinear flight path and net acceleration is normal to the floor of the aircraft (no side-slip vectors).
In mechanical linkage control systems, a coordinated turn required that the pilot simultaneous input (via rudder pedals) the proper amount of yaw to match the amount of roll input provided via the cyclic stick. The more recent fly-by-wire flight control systems (see, e.g., U.S. Pat. Nos. 4,203,532, 4,067,517, 4,206,891 and 4,484,283, all assigned to the assignee of the present invention) automatically provide the matching yaw input. An automatic flight control system (AFCS) commands a coordinated yaw input, typically at air speeds above 60 knots, based on sensed lateral acceleration. The coordinating yaw signal is then used to modify the main rotor and tail rotor command signals as necessary to drive the helicopters lateral acceleration to zero.
U.S. Pat. Nos. 5,213,283, 5,222,691 and 5,238,303, all assigned to the assignee of the present invention, disclose a model following control system for a rotary wing aircraft which provides automatic turn coordination control. In the model following control system disclosed in the above identified patents, automatic turn coordination control is provided in response to a pilot sidearm controller roll input provided that the aircraft bank angle, roll rate and airspeed exceed minimum threshold values. Thereafter, automatic turn coordination is terminated when both bank angle and roll rate are below threshold values.
Under certain flight conditions, automatic turn coordination may not be desirable. This is true in situations where the pilot desires to perform a flat turn which is accomplished by yawing without any roll command, e.g., the pilot is commanding a side slip maneuver. When performing such a side slip maneuver, an inadvertent roll command may automatically place the aircraft into a coordinated turn. An example of a maneuver where a coordinated turn isn't desired is a snap turn. When performing a snap turn, the pilot inputs a yaw command without a corresponding roll command. In response, the aircraft yaws, for example, to aim at a target. Another example of when a pilot would want to inhibit turn coordination is during a take off where there are terrain restrictions so that the pilot takes off laterally then inputs a yaw kick. As the aircraft begins to move laterally from take off, the forward airspeed is approximately zero knots but the bank angle may be above the threshold for automatic turn coordination. When the pilot inputs a yaw command, the aircraft may suddenly meet the 60 knot airspeed requirement and, therefore, turn coordination is automatically engaged and the aircraft will go into a coordinated turn.
One method used to overcome automatic turn coordination is to provide the pilot with a manual switch or button which must be depressed or repositioned to inhibit turn coordination. However, such a manual inhibit greatly increases pilot workload.